Specialized tests for the susceptibility of pathogenic microorganisms are not indicated for most clinical infections. Primary attention should be focused on proper collection of representative culture material and subsequent accurate identification of the offending organism. Specific antimicrobial therapy can then be empirically instituted based upon previously known susceptibility patterns. An overview of the broth dilution, agar dilution, and agar (disk) diffusion techniques is presented. The absolute indications for these tests are as follows: (1) infections with gram-negative bacilli; (2) staphylococcal infections; and (3) enterococcal and other group D beta hemolytic streptococcal infections. The following situations are relative indications for susceptibility testing: (1) infections occurring in immunosuppressed hosts; (2) infections that by their location are more susceptible to certain types of antibiotics; and (3) infections that should be treated with a bactericidal rather than a bacteriostatic drug. The rationale and indications for the serum bactericidal test (SBT or Schlichter test) and antibiotic assay techniques are described. The cases reported illustrate appropriate application of the specialized tests for susceptibility.